The present invention relates to an optical coupler, which is used as an optical source of an optical direct amplifier using the stimulated Raman scatter effect in an optical fiber.
The Raman effect is the phenomenon that when light of the angular frequency (.OMEGA.) illuminates a material, another light of the angular frequency (.OMEGA..+-..DELTA..OMEGA.) is generated depending upon the transition of the transmition of the vibration state and/or the spin state of the material. The light of (.OMEGA.-.DELTA..OMEGA.) is called stokes light, and the light of (.OMEGA.+.DELTA..DELTA.) is called anti-stokes light.
When a strong laser beam of the angular frequency (.OMEGA.) is input to a material, the stokes light of (.OMEGA.-.OMEGA.) is generated through the Raman effect, and the vibration of the frequency (.DELTA..OMEGA.) is generated by the beat between the input beam and the stokes beam. The generated beam triggers another stokes beam, and beat. The stimulated Raman scatter is generated by above process.
An optical direct amplification is carried out by the using the stimulated Raman scatter phenomenon by non-linear effect of an optical fiber. For instance, such an amplifier is reported in No.1115, 4-239 and No.1113, 4-237 in the annual meeting in 1985 of the Institute of Electronics and Communication in Japan.
FIG. 7 shows a prior optical direct amplifier using stimulated Raman scatter effect. In the figure, the numeral 71 is a pump light source, 72 is a light combiner (multiplexer), 73 is a wavelength separator, 74 is an optical fiber for pump light, 75 is an optical fiber for signal, and 76 is an optical fiber for transmission (Raman effect).
In the figure, the pump light in the optical fiber 74 is combined with the signal light in the optical fiber 75. Then, the combined light is transmitted in the optical fiber 76, where the signal light is amplified by the stimulated Raman effect. The wavelength separator 73 separates only the desired wavelength as an amplified output signal.
In the optical direct amplifier using the stimulated Raman effect as shown in FIG. 7, the pump wavelength is selected so that the stokes wavelength generated by the pump wavelength is almost the same as the wavelength of the signal to be amplified. Therefore, a high power pump light source 71 and an optical coupler 72 for coupling both pump light and signal light to a common optical fiber are essential.
Conventionally, a pump light source 71 is a solid laser like a YAG laser. And, an optical combiner 72 for combining a pump light with a signal light is implemented by a half mirror.
By the way, it has been known that silica based optical fiber has small attenuation in the wavelength 1.3 .mu.m and 1.5 .mu.m. In order to amplify those wavelengths, the preferable wavelengths of the pump light source are 1.2 .mu.m and 1.4 .mu.m, respectively.
However, a conventional solid laser, like YAG laser and/or E.sub.r laser can generate only 1.06 .mu.m or 1.55 .mu.m, which is unfortunately different from said preferable wavelength. Further, those solid lasers (YAG laser and E.sub.r laser) has the disadvantage that the size is large and the operation is not easy.
A semiconductor laser which generates said preferable wavelength is now available in the commercial market. A semiconductor laser is small in size as compared with a solid laser, and it is easy to operate and the wavelength is adjustable. However, a semiconductor laser has the disadvantage that the output power is insufficient for a pump light source 71.
The apparatus of FIG. 7 has another disadvantage that the combiner 72 which is implemented by a half mirror has relatively large optical attenuation, since a half mirror reflects a part of pump light and signal light.